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Scientists do not know exactly when this 'cosmic dawn' occurred and whether it was a single, dramatic event that caused all the galaxies to form their first stars, or whether it happened more gradually over millions of years.

The new observations span a period between 350 million and 600 million years after the Big Bang and represent the first reliable census of galaxies at such an early time in cosmic history, the Caltech team says.

The astronomers found that the number of galaxies steadily increased as time went on, supporting the idea that the first galaxies didn’t form in a sudden burst but gradually assembled their stars.

Furthest look achieved yet: The new galaxies are shown at the top of the picture, and their locations are pinpointed in the main image

Because it takes light billions of years to travel such vast distances, astronomical images show how the universe looked during the period, billions of years ago, when that light first embarked on its journey.

The farther away astronomers peer into space, the further back in time they are looking.

The latest study explores the deepest reaches of the cosmos - and therefore the most distant past - that has ever been studied with Hubble.

'We’ve made the longest exposure that Hubble has ever taken, capturing some of the faintest and most distant galaxies,' says Richard Ellis, astronomy professor at Caltech and the first author of the paper.

'The added depth and our carefully designed observing strategy have been the key features of our campaign to reliably probe this early period of cosmic history.'

The results are the first from a new Hubble survey that focused on a small patch of sky known as the Hubble Ultra Deep Field (HUDF), which was first studied nine years ago.

This timeline shows the development of the galaxies in our universe since the Big Bang 13.7billion years ago: Because it takes light billions of years to travel vast distances, the farther away astronomers peer into space, the further back in time they are looking

The astronomers used Hubble’s Wide
Field Camera 3 (WFC3) to observe the HUDF in near-infrared light over a
period of six weeks during August and September 2012.

To
determine the distances to these galaxies, the team measured their
colours using four filters that allow Hubble to capture near-infrared
light at specific wavelengths.

Pushed to its limit: The observations made by the Hubble Space Telescope (pictured) will be expanded on by the planned James Webb Space Telescope

'We
employed a filter that has not been used in deep imaging before, and
undertook much deeper exposures in some filters than in earlier work, in
order to convincingly reject the possibility that some of our galaxies
might be foreground objects,' said team member James Dunlop of the
University of Edinburgh's Institute for Astronomy.

The
filters allowed the astronomers to measure the light that was absorbed
by neutral hydrogen, which filled the universe beginning about 400,000
years after the Big Bang.

As
stars and galaxies started to form roughly 200million years after the
Big Bang they bathed the cosmos with ultraviolet light, which ionised
the neutral hydrogen by stripping an electron from each hydrogen atom.

This so-called 'epoch of reionisation' lasted until the universe was about a billion years old.

If
everything in the universe were stationary, astronomers would see that
only a specific wavelength of light was absorbed by neutral hydrogen.
But the universe is expanding, and this stretches the wavelengths of
light coming from galaxies.

The
amount that the light is stretched - called the redshift - depends on
distance: the farther away a galaxy is, the greater the redshift.

THE HUBBLE SPACE TELESCOPE

Launched from the Space Shuttle in 1990, the Hubble Space Telescope sits 353 miles above the surface of the Earth peering into the cosmos with a view uninhibited by interference from our atmosphere.

Shifting pockets of air distort light from space — that's why stars seem to twinkle when viewed from the ground. Furthermore, the atmosphere blocks some wavelengths of light partially or entirely, making space the only place where it is possible to get a truly clear and comprehensive view of the universe.

In its first 15 years, the telescope recorded over 700,000 images and helped to expand our understanding of star birth, star death, galaxy evolution, and has helped move black holes from theory to fact.

Hubble's large mirror collects light from celestial objects and directs it to the telescope's instruments, the astronomer’s eyes to the universe.

It is set to be replaced by the James Webb Space Telescope, due for launch in 2018, which will orbit a million miles from Earth.

It is hoped that the infrared-optimised Webb will find the first galaxies that formed in the early Universe, connecting the Big Bang to our own Milky Way Galaxy.

As
a result of this cosmic expansion, astronomers observe that the
absorption of light by neutral hydrogen occurs at longer wavelengths for
more distant galaxies.

The
filters enabled the researchers to determine at which wavelength the
light was absorbed; this revealed the distance to the galaxy - and
therefore the period in cosmic history when it is being formed.

Using this technique to penetrate further and further back in time, the team found a steadily decreasing number of galaxies.

'Our
data confirms that reionisation is a drawn-out process occurring over
several hundred million years with galaxies slowly building up their
stars and chemical elements,' said co-author Brant Robertson of the
University of Arizona in Tucson.

'There wasn’t a single dramatic moment when galaxies formed; it’s a gradual process.'

The new observations - which pushed Hubble to its technical limits - hint at what is to come with next-generation infrared space telescopes, the researchers say. To probe even further back in time to see ever more primitive galaxies, astronomers will need to observe in wavelengths longer than those that can be detected by Hubble.

That’s because cosmic expansion has stretched the light from the most distant galaxies so much that they glow predominantly in the infrared. The upcoming James Webb Space Telescope, slated for launch in a few years, will target those galaxies.

'Although we may have reached back as far as Hubble will see, Hubble has, in a sense, set the stage for Webb,' says team member Anton Koekemoer of the Space Telescope Science Institute in Baltimore. 'Our work indicates there is a rich field of even earlier galaxies that Webb will be able to study.'